Process for purifying crude cyanuric acid

ABSTRACT

Process for purifying crude cyanuric acid whereby crude cyanuric acid is digested with an aqueous phosphoric acid solution containing about 10 to about 85% phosphoric acid at a temperature of about 180° to about 220° C under at least the autogenously developed pressure to digest the crude cyanuric acid. A novel, large, free-flowing cyanuric acid product is recovered free of any hard cement-like masses.

This is a division of application Ser. No. 447,824, filed Mar. 4, 1974.

This invention relates to the purification of crude cyanuric acid withaqueous phosphoric acid solutions at high temperatures. This inventionalso relates to the formation of novel, free-flowing cyanuric acidcrystals.

Cyanuric acid has the empirical formula C₃ H₃ O₃ N₃ and is the mainproduct produced by heating urea, biuret or mixtures of both in a kilnat temperatures of about 200° to about 350° C. Unfortunately, theproduct produced is composed of only about 75% cyanuric acid with theremainder of the product containing about 25% ammelide impurities andminor amounts of other impurities such as ammeline. This product mixtureis conventionally referred to as crude cyanuric acid. Since it is quitedifficult to separate the crude cyanuric acid into its component partsin order to recover pure cyanuric acid, various methods have beenproposed to purify the cyanuric acid by converting the impurities intocyanuric acid by acid hydrolysis. This conversion by acid hydrolysis issometimes referred to as the acid digestion process.

The acid digestion process comprises mixing crude cyanuric acid with astrong mineral acid to give a slurry containing 10 to 15% undissolvedsolids. The mineral acids disclosed as being operative are sulfuric,hydrochloric, nitric and phosphoric acid, with sulfuric acid beingpreferred. The slurry is digested at reflux temperatures (about 104° C)for 1 to 10 hours. This digestion in hot mineral acid results in thehydrolysis of most of the impurities to cyanuric acid. Methods employingthis procedure are described in U.S. Pat. Nos. 2,943,088 and 3,107,244.Higher temperatures, up to about 130° C, may be employed ifsuperatmospheric pressures up to 100 p.s.i. are employed. See U.S. Pat.No. 3,107,244. The use of temperatures above 165° C has also beensuggested with strong mineral acids, such as sulfuric acid, in U.S. Pat.No. 2,768,167.

Because the acid digestion process involves heating and mixing a thickslurry of solids in a digester vessel for long periods of time, someproblems are encountered. Mixing in the digester vessel is oftendifficult and a constant build-up of solids on the walls of the digesterresults. Frequently, large chunks of this solid build-up break away fromthe walls and either plug the exit lines or bend the agitator.Furthermore, strong mineral acid reaction mixtures held at operatingtemperatures for prolonged periods can result in partial hydrolysis ofthe cyanuric acid to ammonia and carbon dioxide, thus decreasingcyanuric acid yields.

Additional problems result when hydrochloric acid or phosphoric acidhave been employed at conventional operating temperatures rendering theuse of either of these acids commercially impractical. Commercialoperation of a hydrochloric acid digestion process is extremelydifficult and hazardous and results in many shut downs because ofequipment breakdown and corrosion. Commercial operation of a phosphoricacid digestion process has not been heretofore possible because of theslow impurity conversion rate and because the cyanuric acid is partiallyhydrolyzed to ammonia and carbon dioxide over the prolonged digestionperiod. Furthermore, the reaction rate with phosphoric acid isapproximately five times slower than with strong mineral acids, such assulfuric acid. The use of higher temperatures, up to about 130° C, bythe prior art, has not alleviated these difficulties.

The sulfuric acid and nitric acid digestion processes, while beingcommercially effective processes, have caused numerous pollutionproblems with regard to the separated acid digestion solution. Theseparated acid digestion solution has been partially or entirelydiscarded as an untreated waste stream since treatment to render themsafe for discharge is difficult and expensive. Untreated acid digestionsolutions discharged into natural water ways, however, disrupts andoften destroys natural fauna and flora.

In addition to the above acid digestion process deficiencies, thecyanuric acid product produced by these processes is often difficult toseparate from the acid digestion solution and difficult to handle onceseparated because of the samll crystals produced. The crystals generallyhave particle sizes between about 30 and 50 microns. Crystals of thissize must be filtered carefully to prevent valuable cyanuric acidcrystals from passing through conventional separating means along withthe filtrate.

It has been unexpectedly discovered that crude cyanuric acid can bepurified by mixing sufficient amounts of crude cyanuric acid with anaqueous phosphoric acid solution containing about 10 to about 85%phosphoric acid to form a 10 to 45% crude cyanuric acid slurry, heatingthe slurry to a temperature of about 180° to about 220° C under at leastthe autogenously developed pressure to digest the crude cyanuric acid,cooling the digested cyanuric acid to precipitate the cyanuric acidcrystals, and recovering the precipitated cyanuric acid crystals.

The process of this invention permits the purification of crude cyanuricacid in a commercially simple and efficient manner without the need forlarge expensive digester reactors. It permits the recovery of cyanuricacid in exceptionally high yields and exceptionally high purities inrelatively short periods of time, that is in less than 60 seconds. Itunexpectedly results in the formation of a cyanuric acid product whichcontains at least 90% cyanuric acid crystals having particle sizesbetween 60 and 650 microns. Furthermore, the invention permits thetreatment of the acid digestion solution effluent in a simple andefficient manner to produces useful by-products.

In the drawings,

FIG. 1 is a photomicrograph of cyanuric acid crystals formed from crudecyanuric acid digested with phosphoric acid according to the invention.

FIG. 2 is a photomicrograph of cyanuric acid crystals formed from crudecyanuric acid digested with sulfuric acid.

In the process of the invention, crude cyanuric acid containing ammelideand other minor impurities is mixed with an aqueous phosphoric acidsolution. The aqueous phosphoric acid solution dissolves allacid-soluble impurities present in the reaction mixture. It hydrolyzesammelide to yield cyanuric acid and a corresponding ammonium salt, thatis, ammonium phosphate. Additionally, ammonium cyanurates are hydrolyzedto yield cyanuric acid and the corresponding ammonium salt.

The crude cyanuric acid is employed in amounts sufficient to produce acrude cyanuric acid slurry. The crude cyanuric acid slurry concentrationis not critical. However, from a commercial process standpoint, slurryconcentrations below about 10% or above about 45% are not desirable.Slurry concentrations below about 10% are not economical in view of thesmall cyanuric acid concentrations treated. Slurry concentrations aboveabout 45% are not workable because they are difficult to handle.Accordingly, crude cyanuric acid slurry concentrations between 10 and45% are employed, with slurry concentrations between 15 and 25% beingpreferred.

The aqueous phosphoric acid solution must be employed in concentrationsof about 10 to about 85% by weight phosphoric acid. As the phosphoricacid concentration drops below about 10% or increases above about 85%,the rate of conversion of the cyanuric acid impurities into cyanuricacid decreases sharply, becoming negligible in pure water or in purephosphoric acid (100%). The aqueous phosphoric acid solution is preparedfrom orthophosphoric acid, pyrophosphoric acid, super-phosphoric acid orcombinations thereof dispersed in water to the desired phosphoric acidconcentration.

Digestion of the crude cyanuric acid slurry must be carried out at atemperature of about 180° to about 220° C. At temperatures of about 180°to about 220° C, the reaction rate is very rapid and substantially allof the cyanuric acid impurities are converted to cyanuric acid. Attemperatures below about 180° C, the reaction rate is slow and theamount of ammelide converted to cyanuric acid is significantlydecreased. At temperatures above about 220° C, phosphoric acid catalyzesthe thermal decomposition of cyanuric acid, thus substantiallydecreasing cyanuric acid yields. Optimum reaction rate and optimumammelide conversion occurs at the preferred temperatures of about 195°to about 217° C.

Digestion of the crude cyanuric acid slurry must be carried out underpressure in order to prevent water vaporization losses. The pressure,however, is not critical and the autogenously developed pressure at thevarious reaction temperatures is normally used. Generally, theautogenously developed pressure will vary from about 130 to about 275p.s.i.g. at reaction temperatures of about 182° to about 217° Crespectively.

The time period required for the reaction to be maintained at thedesired operating temperature is not critical. Once the reaction massreaches the particular operating temperature, the crude cyanuric acidimpurities immediately begin forming cyanuric acid. Maximum conversion,that is over 90%, of the cyanuric acid impurities to cyanuric acid havebeen obtained in the laboratory in reaction times up to about 60seconds. Reactiom times longer than 60 seconds have not significantlyincreased the percentage of impurity converted. However, from acommercial standpoint, reaction times up to about 10 minutes andpreferably 1 to 5 minutes are employed when conventional pressurereactors are used. Shorter reaction times, that is reaction times up to60 seconds, may be commercially feasible with commercially availablepipe reactors. A pipe reactor is an elongated tubular reaction chamberwherein the feed enters the reactor in one end and exits out the otherend. The reaction takes place within the tube which is heated byexternal sources. Use of pipe reactors greatly increases the productionof purified cyanuric acid and eliminates the need for large, expensivereactors currently used.

The use of phosphoric acid to purify crude cyanuric acid in high yieldsat high reaction temperatures, and in relatively short reaction periodsis highly unexpected. It is unexpected because phosphoric acid is arelatively weak mineral acid as is apparent from its dissociationconstants (pK₁ is 2.12, pK₂ is 7.21, and pK₃ is 12.32) as compared tothe fully dissociable strong mineral acids conventionally employed, suchas sulfuric acid (pK₁ is 0.40 and pK₂ is 1.92), hydrochloric acid andnitric acid. It is also unexpected since only one hydrogen ion isavailable from phosphoric acid for catalyzing the hydrolysis of ammelidewhereas two hydrogen ions are available from sulfuric acid. Furthermore,it was unexpected because the ionization of K₁ is not appreciablyincreased as a function of temperature. In contrast, the ionization ofthe strong mineral acids is increased at increased reactiontemperatures.

Mixing of the crude cyanuric acid with the aqueous phosphoric acidsolution and heating mixture to the desired operating temperature areachieved by conventional means and procedures. Mixing and heating may bedone separately or carried out in a single stage. For example, whenmixing and heating are done separately, the crude cyanuric acid is mixedwith water to form a slurry of cyanuric acid, the slurry is placed in apressure vessel and heated to the desired temperature. The aqueousphosphoric acid solution is then passed into the pressure vessel, mixedwith the crude cyanuric acid, and the reaction takes place. When mixingand heating are carried out in a single stage process, crude cyanuricacid, in either dry, moist or water slurried form is added to theaqueous phosphoric acid solution, mixed, and passed into a reactor whichis previously or subsequently heated to the desired temperature. Thereaction is then permitted to go to completion. Alternate procedures maylikewise be employed.

When the acid digestion reaction is complete, the hot digested cyanuricacid is cooled by any conventional means to precipitate the cyanuricacid crystals. The crystals are then recovered from the acid digestionsolution by any desirable means. One process that may be employed torecover the cyanuric acid crystals is disclosed in U.S. Pat. No.3,107,244. In this process the acid digestion solution is cooled to atemperature above about 57° C to precipitate anhydrous cyanuric acidcrystals. The precipitated crystals are then separated from thedigestion solution by filtration at a temperature above about 57° C. Theseparated crystals are then washed with hot water at a temperature aboveabout 57° C, and the washed cyanuric acid crystals are recovered.Alternative methods for precipitating and recovering cyanuric acidcrystals may also be employed.

The recovered cyanuric acid crystals may then be dried and stored, orpassed directly to a chlorinator and converted into chloroisocyanuricacids. The conversion of cyanuric acid into chloroisocyanuric acids,such as dichloroisocyanuric acid and/or trichloroisocyanuric acid iswell known in the art and does not constitute part of this invention.

Drying may be carried out in any conventional manner in order to removeresidual moisture and to produce a free-flowing crystalline product.Preferably, the crystals are heated to temperatures of at least 120° C.

Removal of the digestion solution from the cyanuric acid crystalsresults in crystals that can be handled easily and prevents theformation of hard cement-like masses of cyanuric acid. However, removalof all of the digestion solution from the crystals is not commerciallyfeasible. In has been determined that removal of all but residual traceamounts of digestion solution from the crystals produces a commerciallysatisfactory product. These residual trace amounts of digestion solutionremaining on the crystals must generally constitute less than 0.1% byweight, and preferably about 0.01 to about 0.08% by weight phosphatevalues (phosphoric acid and phosphate salts, such as ammoniumphosphate). These minor traces of digestion solution do not adverselyaffect the quality or utility of the cyanuric acid crystals.

The separated digestion solution contains most of the dissolvedimpurities, ammonium salts, and excess phosphoric acid. The entiredigestion solution or portions thereof may be sporadically orcontinuously recycled to the digester. The portion of the digestionsolution not recycled is conveniently treated to produce phosphatecontaining compounds. The most economical phosphate containing compoundsto produce are ammonium phosphate compounds which are readily utilizedas fertilizers. Processes for producing ammonium phosphate fertilizersare well known and are described in the literature. By treating thedigestion solution in this manner, pollution abatement is readilyachieved and vital by-products are produced.

The process of the invention may be carried out in a batch type manneror continuously with or without recycle. A once-through acid digestionsystem is preferable to a recycle system since the former systemimproves control over the entire process by eliminating the maintenancenecessary to monitor and adjust phosphoric acid concentrations duringdigestion of the crude cyanuric acid.

The cyanuric acid crystals produced according to the process of thisinvention are novel, large, well-defined, free-flowing crystals. Crystalsize has been unexpectedly discovered to be a function of phosphoricacid concentration. For example, when the aqueous phosphoric acidsolution contains about 10 to about 30% phosphoric acid, at least 90% ofthe cyanuric acid crystals recovered have particle sizes between 400 and650 microns. When the aqueous phosphoric acid solution contains about 30to about 85% phosphoric acid, at least 90% of the cyanuric acid crystalsrecovered have particle sizes between 60 and 100 microns. These crystalsare 2 to 8 times larger than conventionally prepared cyanuric acidcrystals. The direct formation of large crystals permits the crystals tobe recovered in an easy and efficient manner without the need forcareful controls to prevent cyanuric acid losses formerly attributableto conventional separating equipment. Furthermore, the crystals producedaccording to the present invention have frangibility valuessubstantially equal to conventionally prepared crystals. This propertypermits the crystals to be handled, shipped and stored easily withoutthe difficulties associated with dusting.

The invention will be better understood from a consideration of thefollowing examples, and a reference to the attached illustrations. Theexamples are given to illustrate the invention, and are not deemed to belimiting thereof. All percentages given are based upon weight unlessotherwise indicated. The illustrations are reflective photomicrographsof cyanuric acid crystals which clearly demonstrate the inventiondescribed and claimed herein, and are identified as follows.

FIG. 1 shows crystals of cyanuric acid formed by purifying crudecyanuric acid with phosphoric acid. The photomicrograph was taken at 15X magnification.

FIG. 2 shows crystals of cyanuric acid formed by purifying crudecyanuric acid with sulfuric acid. The photomicrograph was taken at 15 Xmagnification.

EXAMPLE 1

This example demonstrates the effect temperature has on the percentageof ammelide converted to cyanuric acid.

Inventive Runs 1, 2, and 3

A 25.8 gram (0.2 mole) sample of crude cyanuric acid prepared from ureaassaying about 80% cyanuric acid and about 20 % ammelide was mixed with92.1 grams of water to form a 17.3% slurry. The slurry was charged intoa 300 milliliter Hastelloy B autoclave. The autoclave was sealed,shaken, and heated to a temperature of approximately 6° C less than thereaction temperatures recited in Table I, Runs 1, 2, and 3. A 27.9 gramsample of 20% orthophosphoric acid was then blown into the autoclaveunder pressure. The phosphoric acid addition took place in less than 2seconds. Upon acid addition, the reaction temperature immediately roseto the temperatures recited in Table I, Runs 1, 2, and 3. The reactiontemperature was maintained for one minute. The autoclave was thenquenched in an ice bath and rapidly cooled to 80° C within 30 seconds.The reaction mixture was removed from the autoclave at room temperatureand the crystallized cyanuric acid was filtered from the slurry. Thefiltered cyanuric acid crystals and separated filtrate was then analyzedfor cyanuric acid and ammelide content. The results are set forth inTable I.

Comparative Run A

A 25.8 gram (0.2 mole) sample of crude cyanuric acid prepared from ureaassaying about 80% cyanuric acid and about 20% ammelide was mixed with27.9 grams of 20% orthophosphoric acid. The slurry was charged into a200 milliliter three-necked flask equipped with a magnetic stirrer,thermometer, and condenser, and refluxed at 104° C for 240 minutes. Themixture was then cooled rapidly and filtered according to Example 1, andthe filtered cyanuric acid crystals and separated filtrate was thenanalyzed for cyanuric acid and ammelide content. The results are setforth in Table I.

                                      TABLE I                                     __________________________________________________________________________                Reaction        Total Analysis                                                                            %                                            Time Temperature                                                                          Reaction Pressure                                                                      % Cyanuric                                                                          %     Ammelide                              Run No.                                                                              (minutes)                                                                          ° C                                                                           p.s.i.g. acid  Ammelide                                                                            Conversion                            __________________________________________________________________________    Run 1  1    182    130      >99.9 trace 90.1                                  Run 2  1    196    200      >99.9 trace 96.4                                  Run 3  1    206    240      >99.9 trace 99.0                                  Comparative                                                                   Run A  240    104° C                                                                      0         92.1 7.9   46.5                                  __________________________________________________________________________     ">" means greater than                                                   

EXAMPLE 2

This example demonstrates the effect of reaction time on the percentageof ammelide converted to cyanuric acid.

Inventive Runs 4 and 5

The procedure of Example 1, Run 1 was repeated in Run 4 except that thereaction temperature was maintained at 182° C for 2 minutes. Theprocedure of Example 1 was repeated in Run 5 except that the sealedautoclave was heated to 198° C and the phosphoric acid addition tookplace in less than 6 seconds, and the reaction temperature wasmaintained for 5 seconds after phosphoric acid addition. At the end of 5seconds, the autoclave was cooled and the contents removed, separatedand analyzed according to Example 1. The results are set forth in TableII.

                                      TABLE II                                    __________________________________________________________________________              Time Reaction Total Analysis % Ammelide                             Example                                                                            Run No.                                                                            (seconds)                                                                          Temperature ° C                                                                 % Cyanuric Acid                                                                       % Ammelide                                                                           Conversion                             __________________________________________________________________________    1    1     60  182      >99.9   trace  90.1                                   2    4    120  182      >99.9   trace  90.5                                   2    5     5   198      >99.9   trace  91.3                                   __________________________________________________________________________     ">" means greater than                                                   

EXAMPLE 3

This example demonstrates the effect of phosphoric acid concentrationand types of phosphoric acid employed on the percentage of ammelideconverted to cyanuric acid. This example also demonstrates the effect ofphosphoric acid concentration on cyanuric acid crystal size.

Inventive Runs 6 through 12

The procedure of Example 1 was repeated at a reaction temperature of197° C and pressure of 180 p.s.i.g. with various concentrations of orthoand superphosphoric acid. The results are set forth in Table III.

Comparative Runs B and C

The procedure of Example 1 was repeated except in comparative Run B nophosphoric acid was added and in comparative Run C a solution of 100%phosphoric acid was added. The results are set forth in Table III.

                                      TABLE III                                   __________________________________________________________________________                  %               %                                                             H.sub.3 PO.sub.4                                                                     Form of  Ammelide                                                                            Cyanuric Acid                             Run No.                                                                              Time (min.)                                                                          Concentration                                                                        Phosphoric Acid                                                                        Conversion                                                                          Crystal Size                              __________________________________________________________________________    6      1      11     ortho    96.7  > 90% of crystals re-                                                         covered had particle                                                          sizes between 400 and 650                                                     microns                                   7      1      20     ortho    96.4  same as Run 6                             8      1      50     ortho    95.5  > 90% of crystals re-                                                         covered had particle                                                          sizes between 60 and 100                                                      microns                                   9      1      65     ortho    98.2  same as Run 8                             10     1      65      super*  98.0  same as Run 8                             11     1      75     ortho    95.5  same as Run 8                             12     1      75      super*  95.6  same as Run 8                             Comparative                                                                   B      1       0     none      0.0  30 to 50 microns                          C      1      100     super*   0.0  30 to 50 microns                          __________________________________________________________________________     ">" means greater than                                                        *mixture of pyro and ortho                                               

EXAMPLE 4

This example compares cyanuric acid crystals produced according to thepresent invention with cyanuric acid crystals produced under identicalconditions with sulfuric acid.

Inventive Run 13

A 25.8 gram (0.2 mole) sample of crude cyanuric acid prepared from ureaassaying about 80% cyanuric acid and about 20% ammelide was mixed with92.1 grams of water to form a 17.3% slurry. The slurry was charged intoa 300 milliliter Hastelloy B autoclave. The autoclave was sealed,shaken, and heated to 192° C. A 27.9 gram sample of 20% phosphoric acidwas then blown into the autoclave under pressure. The phosphoric acidaddition took place in less than 2 seconds. Upon acid addition, thereaction temperature immediately rose to 198° C. The reactiontemperature was maintained for one minute. The autoclave was thenquenched in an ice bath and rapidly cooled to 80° C within 30 seconds.The reaction mixture was removed from the autoclave at room temperatureand the crystallized cyanuric acid was filtered from the slurry. Therecovered cyanuric acid crystals are illustrated in FIG. 1. The producthad a bulk density of 32 lb/ft³ and had at least 90% cyanuric acidcrystals with particle sizes between 400 and 650 microns.

Comparative Run D

The procedure of Inventive Run 13 was repeated except that 20% sulfuricacid was used instead of 20% phosphoric acid. The cyanuric acid cyrstalsare illustrated in FIG. 2. The product had a bulk density of 37 lb/ft³and had at least 90% cyanuric acid crystals with particle sizes between100 and 200 microns. The cyanuric acid crystals produced from InventiveRun 13 and Comparative Run D both had substantially identicalfrangibilities.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications are intended to be included within the scope of thefollowing claims.

What is claimed is:
 1. A novel free-flowing cyanuric acid compositionhaving at least 90% cyanuric acid crystals with particle sizes between400 and 650 microns and containing about 0.01 to about 0.08% phosphatevalues produced by mixing sufficient amounts of crude cyanuric acid withan aqueous phosphoric acid solution containing about 10 to about 30%phosphoric acid to form a 10 to 45% crude cyanuric acid slurry;heatingthe slurry to a temperature of about 180° to about 220° C under at leastthe autogenously developed pressure to digest the crude cyanuric acid;cooling the digested cyanuric acid to precipitate the cyanuric acidcrystals; and recovering the novel cyanuric acid composition.